Security is an important requirement in many document production applications. In situations such as official or government document printing, event ticket printing, financial instrument printing and the like, many documents must be protected against copying, forging and/or counterfeiting. To do this, printed documents often include security marks that serve to prevent counterfeiting and/or identify a document as original.
A correlation mark is one example of a security mark. A correlation mark includes one or more elements that are invisible or hardly visible on a printed document. The hidden elements become visible when the document is viewed through a correlation mask that serves as a “key” to the correlation mark. An example of a correlation mark is taught in U.S. Pat. No. 5,734,752 to Knox, titled “Digital watermarking using stochastic screen.” Other examples are disclosed in U.S. Pat. No. 7,894,626 to Wang et al., titled “System and method for the generation of multiple angle correlation-based digital watermarks,” as well as in U.S. Pat. No. 7,894,626 to Wang et al., titled “System and method for the generation of multiple angle correlation-based digital watermarks” patterns.” The disclosures of each of these references are fully incorporated herein by reference.
Although security markings such as correlation marks are effective on printed documents, they require a special device (a “screen” or “key”) to decode them.
This document describes methods and systems for decoding a security mark that addresses at least some of the problems described above, and/or other problems.